How to determine in filter(struct __sk_buff *skb) if there is an ethernet header

I'm building a pcap filter which uses the 'filter' function to determine if a packet should be sent to user space. This works fine if I attach it to an ethernet interface. If I attach the same function to a GRE interface, then it doesn't work as the packet that arrives doesn't have an ethernet header on it.

I can't figure out from the fields in the __sk_buff whether there is a way of telling if there is an ethernet header or not. Any ideas? All the examples seem to assume that there is an ethernet header.

My fallback is to get the interface type and specifically check for GRE.... But this doesn't seem scalable.

Start with the filter function and the __sk_buff fields referenced in the report, then compare packet layouts on Ethernet and GRE interfaces. Check whether existing examples or documentation explain how to distinguish the headers without interface-specific checks; done means the determination path or limitation is clearly documented.

Written by the indexing model from the issue text.